One difficulty in using thermally activated free radical initiators, e.g., organic peroxides and azo compounds, in crosslinking, i.e., curing, elastomeric and thermoplastic materials is that they may initiate premature crosslinking, i.e., scorch, during compounding and/or processing prior to the actual phase in the overall process in which curing is desired. With conventional methods of compounding, such as milling, Banbury, or extrusion, scorch occurs when the time-temperature relationship results in a condition in which the free radical initiator undergoes thermal decomposition which, in turn, initiates a crosslinking reaction that can create gel particles in the mass of the compounded polymer. These gel particles can adversely impact the homogeneity of the final product. Moreover, excessive scorch can so reduce the plastic properties of the material that it cannot be efficiently processed with the likely possibility that the entire batch will be lost.
One widely accepted method for minimizing scorch is to choose a free radical initiator that has a sufficiently high activation temperature so that compounding and/or other processing steps can be successfully completed prior to the final curing step. Typical of this class of initiators are those with a high 10-hour half-life temperature. The disadvantages of this method are longer cure times, and thus lower throughput. Higher cure temperatures can be used to offset the longer cure times, but then higher energy costs are incurred. Higher cure temperatures can also adversely affect the thermal stability of the materials.
Another method of minimizing scorch is to lower the compounding and/or processing temperature to improve the scorch safety margin of the crosslinking agent. This method, however, may have limited scope depending upon the polymer and/or process involved. In addition, here too curing at a lower temperature requires a longer cure time and results in lower throughput. Lower temperatures can also increase the viscosity of the material which in turn can make mixing more difficult, and can increase the risk of running up against the freezing point of the polymer.
Yet another method of minimizing scorch is the incorporation of scorch inhibitors into the compositions, For example, British patent 1,535,039 discloses scorch-resistant compositions comprising organic hydroperoxides and ethylene polymers. U.S. Pat. No. 3,751,378 discloses the use of N-nitroso diphenylamine or N,N′-dinitroso-para-phenylamine as scorch retardants incorporated into a polyfunctional acrylate crosslinking monomer for providing long Mooney scorch times in various elastomer formulations. U.S. Pat. No. 3,202,648 discloses the use of nitrites such as isoamyl nitrite, tert-decyl nitrite and others as scorch inhibitors for polyethylene. U.S. Pat. No. 3,954,907 discloses the use of monomeric vinyl compounds as protection against scorch. U.S. Pat. No. 3,335,124 describes the use of aromatic amines, phenolic compounds, mercaptothiazole compounds, bis(N,N-disubstituted-thiocarbamoyl) sulfides, hydroquinones and dialkyldithiocarbamate compounds. U.S. Pat. No. 4,632,950 discloses the use of mixtures of two metal salts of disubstituted dithiocarbamic acid in which one metal salt is based on copper.
One commonly used scorch inhibitor for use in free radical, particularly peroxide, initiator-containing compositions is 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl also known as nitroxyl 2, or NR 1, or 4-oxypiperidol, or tanol, or tempol, or tmpn, or probably most commonly, 4-hydroxy-TEMPO or even more simply, h-TEMPO. The addition of 4-hydroxy-TEMPO minimizes scorch by “quenching” free radical crosslinking of the crosslinkable polymer at melt processing temperatures, but it also decreases rate and degree of crosslinking at subsequently elevated (vulcanization) temperatures. Furthermore, 4-hydroxy-TEMPO has a relatively low molecular weight and as such, it is relatively volatile and this can lead to loss during processing and build-up on equipment due to sublimation.
Another problem with this nitroxide scorch inhibitor is its incompatibility with many common polymers that are crosslinked with the help of one or more peroxide initiators. These polymers include ethylene-propylene-diene monomer (EPDM), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), ethylene vinyl acetate (EVA) with less than about 10 wt % vinyl acetate content, unsaturated acrylates, e.g., methyl, ethyl or butyl acrylate with less than about 10 wt % acrylate content, and the like. As a result of this incompatibility, 4-hydroxy-TEMPO migrates to the surface of a pelleted or sheeted product and can cause handling issues such as dusting, segregation, stickiness and the like. In addition, the product can lose its scorch inhibition efficiency over time, and thus its shelf-life can be compromised. This migration problem usually manifests itself when the nitroxide is blended with the polymer to make a concentrate or masterbatch, and it can also limit the concentration of the nitroxide in a fully formulated product.